1. Field of the Invention
The present invention relates to a system based on broadcast/communication convergence, and more particularly to a TDMA system including an optical network unit for multiplexing and transmitting broadcast data and Ethernet data and a subscriber set-top box for receiving and processing the multiplexed data.
2. Description of the Related Art
Conventionally, broadcast transmission is unidirectional transmission that is received by a number of unspecified receivers according to the characteristics of the broadcast. Service and network providers for the broadcast are typically identical. Communication, on the other hand, is bidirectional and is based on one-to-one transmission and reception operations according to the characteristics of the communication. In this case, service and network providers for the communication typically are not identical.
Furthermore, not only are the broadcast and communication service providers different, but the relay networks for the broadcast and communication transmission are also different. For this reason, the linkage of the broadcast and communication transmission has not been considered. However, as transmission networks are quickly developed and the boundary between broadcast and communication has become less distinct, interest in a system based on broadcast/communication convergence has increased.
In order for quality of service (QoS) of communication data to be ensured when the broadcast and communication transmission are converged, the communication data and broadcast data are time division multiplexed and the broadcast data is transmitted in the form of a multi-program transport stream (MPTS). In case of an Ethernet communication service, Ethernet data is transmitted in packets. Thus, a broadcast data transmission method and a communication data transmission method are different and must be accommodated in a system based on or providing broadcast/communication convergence.
In the case of a broadcast transmission, a seamlessly continuous signal must be provided. Thus, the broadcast data must be transmitted in packets wherein preceding and following packets can be linked to each other after the packetizing operation. It is thus necessary that a buffer and delay time are required for the packet processing operation. A processing operation based on the time division multiplexing is required wherein the broadcast data is provided by a seamlessly continuous signal in real time.
A communication network using conventional Ethernet equipment is a packet communication network, and is based on a local area network (LAN) or Internet communication network. A configuration of the Ethernet communication network used by subscribers typically uses a 10 Megabit/sec transmission (referred to as 10 Mbase-T). This is used in many LANs and is used in companies and schools. Recently, a 100-Mbps Internet communication service of 100 Mbase-T is being provided to subscriber homes using an unshielded twisted pair (UTP) cable referred to as a metro Ethernet communication network. Typically, an Ethernet signal is a signal based on a unit of a packet consisting of frames. In order for the Ethernet signal to be multiplexed into a continuous bit stream, an operation for processing various information units such as packet information is needed. For this reason, overhead (O/H) increases and an additional operation must be carried out.
Systems based on the broadcast/communication convergence using the Ethernet communication network and Ethernet equipment are being developed. In such system, a time division multiplexer (TDM) multiplexes a digital broadcast stream and Ethernet data, and a new form of a digital stream is generated and distributed to the subscribers.
FIG. 1 illustrates a conventional system for providing a broadcast/communication convergence service. As shown, the system includes an optical line terminal (OLT) 10 for electro-optically converting broadcast data from a broadcast provider (not shown) and transferring the electro-optically converted broadcast data through an optical signal that can be transmitted and provided to subscribers, an optical network unit (ONU) 20 serving as a user side device for transferring information received from the OLT 10 to a subscriber set-top box 30, and an optical cable connected between the OLT 10 and the ONU 20. When the broadcast/communication data from a service provider (not shown) is transmitted to the ONU 20 through the OLT 10, the ONU 20 receives a service request from a terminal of a service user and carries out a time division multiplexing operation for the broadcast/communication data, such that a corresponding service is provided.
FIG. 2 is a block diagram illustrating an example of a conventional time division multiplexing and demultiplexing operation in a broadcast/communication convergence system. As shown, the broadcast/communication convergence system multiplexes broadcast signals and a communication signal into a single signal according to known time division multiplexing methods, and then transmits the multiplexed single signal. For the system shown, the system converts the communication signal into a media independent interface (MII) signal that is equivalent to a broadcast stream by means of a physical layer (PHY) transceiver 23. The converted communication signal and the broadcast signals are applied to a time division multiplexer (TDM) 21 such that the TDM 21 carries out the time division multiplexing operation on the inputted signals and causes to be transmitted the single signal 25. The Ethernet MII signal is 4-bit parallel signal that are discontinuously received with packet lengths varying between a minimum of 64 bytes and a maximum of 1518 bytes.
At the receiving side, the received signal is provided to time division demultiplexer (TDDM) 31 so that the TDDM 31 carries out a time division demultiplexing operation. Broadcast streams, based on each broadcast channel, and the MII signal, i.e., the communication signal, are outputted as a result of the time division demultiplexing operation.
When the broadcast/communication convergence system transmits an optical signal to a customer, e.g., a consecutive digital broadcast or Video on Demand (VoD) signal, the transmission is unidirectionally, and any Ethernet communication signal is transmitted and received in a burst mode. Thus, because the Ethernet packet length is variable, information of an Ethernet packet length is extracted and an idle signal is generated for a section in which Ethernet data is not present. Thus, a buffering operation is required to be carried out before the multiplexing operation is performed.
Furthermore, when the ONU 20 multiplexes 5-bit Ethernet packet data and a broadcast digital stream and transmits the resultant multiplexed signal, the Ethernet data must be converted into, for example, 4-bit data (100M) based on a media access control (MAC) format having xMII through the PHY transceiver 23. For this, when reaching a MAC layer (data link layer) through a physical layer (PHY) of the seven layer Open Systems Interconnection (OSI) model, the Ethernet data encoded into 5-bit data (125M) must be converted into 4-bit data (100M). Here, the 5-bit data includes control codes indicating a start and end of the data packet or is converted into the 4-bit data so that true Ethernet data can be generated. When a packet with the true Ethernet data is transmitted, the data link layer cannot recognize the start and end of a packet as long as Internet protocol (IP) information of a data field in a frame format, as shown in FIG. 3, is not analyzed.
A digital logic device such as a field programmable gate array (FPGA) can further be provided downstream of the PHY transceiver so that a 4-bit code can be encoded into a 5-bit code. In this case, the 5-bit code in the FPGA can use 16 control codes, but the bandwidth is increased by 25%. When an Ethernet time division multiplexing operation for the 5-bit code is carried out, the bandwidth required is excessively increased. Hence, the 5-bit code is not appropriate for a relatively slow logic device such as the FPGA.
Hence, when the time division multiplexing operation for 4-bit MAC data and a digital stream is desired to be carried out, a need exists for an additional device capable of analyzing IP information in the data link layer and analyzing packet length information.